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Magnetic Fields of M Dwarfs

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Magnetic Fields of M Dwarfs Astron Astrophys Rev (2021)29:1 https://doi.org/10.1007/s00159-020-00130-3(0123456789().,-volV)(0123456789().,-volV) REVIEW ARTICLE Magnetic fields of M dwarfs Oleg Kochukhov1 Received: 22 June 2020 / Accepted: 5 November 2020 Ó The Author(s) 2020 Abstract Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect sig- natures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields. Keywords Stars: activity Á Stars: atmospheres Á Stars: interiors Á Stars: low mass Á Stars: magnetic field Á Stars: rotation Á Techniques: polarimetric Á Techniques: spectroscopic Contents 1 Introduction............................................................................................................................... 2 Methods of magnetic field measurements............................................................................... 2.1 Zeeman effect in spectral lines ....................................................................................... 2.2 Local Stokes parameter spectra ...................................................................................... 2.3 Disk-integrated Stokes parameters.................................................................................... & Oleg Kochukhov [email protected] 1 Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden 123 1 Page 2 of 61 O. Kochukhov 2.4 Zeeman broadening and intensification ............................................................................ 2.5 Least-squares deconvolution ............................................................................................. 2.6 Zeeman Doppler imaging.................................................................................................. 2.7 Instrumentation for magnetic field measurements ........................................................... 3 Observations of M-dwarf magnetic fields................................................................................. 3.1 Total magnetic fields from intensity spectra.................................................................... 3.1.1 Results from detailed line profile modelling ................................................... 3.1.2 Approximate measurements of average magnetic fields ................................. 3.1.3 Magnetic field and stellar rotation ................................................................... 3.2 Large-scale magnetic fields from spectropolarimetry...................................................... 3.2.1 Polarisation in M-dwarf spectra ....................................................................... 3.2.2 Zeeman Doppler imaging results...................................................................... 3.2.3 Comparison of global and total magnetic fields.............................................. 3.2.4 Extended stellar magnetospheres...................................................................... 4 Outlook and discussion .............................................................................................................. 4.1 Summary of observational results..................................................................................... 4.2 Theoretical dynamo models .............................................................................................. 4.3 Magnetic stellar structure models ..................................................................................... 4.4 Future research directions ................................................................................................. Appendix 1: Summary of M-dwarf magnetic field strength measurements using Zeeman broadening and intensification ......................................................................................................... Appendix 2: Summary of Zeeman Doppler imaging results for M dwarfs................................... Appendix 3: Spectral types and rotation periods of M dwarfs with magnetic field measurements References......................................................................................................................................... 1 Introduction M dwarfs are the lowest mass stars, occupying the bottom of the main sequence. These stars dominate the local stellar population, accounting for 70–75% of all stars in the solar neighbourhood (Bochanski et al. 2010; Winters et al. 2019). M dwarfs have masses of 0.08–0.55 M and effective temperatures of 2500–4000 K (Pecaut and Mamajek 2013). Their atmospheric characteristics span a wide range, from conditions similar to the upper layers of GK-star atmospheres in early M dwarfs to temperatures and pressures comparable to those found in brown dwarfs and giant planets in late-M stars. The optical and near-infrared spectra of M dwarfs are distinguished by prominent absorption bands of diatomic molecules. This molecular absorption becomes progressively more important towards later spectral types, to the extent that hardly any atomic line is free from molecular blends. The interior structure of M dwarfs undergoes a transition at M 0:35M (Chabrier and Baraffe 1997) from being similar to that of solar-like stars, with a thick convective envelope overlaying a radiative zone, to a fully convective interior structure not found in any other type of main sequence stars. M dwarfs exhibit conspicuous and abundant evidence of surface activity: flares, photometric rotational variability, and enhanced chromospheric and coronal emission in X-rays, UV, and radio (e.g. Hawley et al. 2014; Newton et al. 2016, 2017; Astudillo-Defru et al. 2017; Wright et al. 2018; Villadsen and Hallinan 2019). In hotter stars, including the Sun, all these phenomena are invariably correlated with the presence of intense magnetic fields generated by a dynamo 123 Magnetic fields of M dwarfs Page 3 of 61 1 mechanism. It is believed that the dynamo process in solar-type stars is closely linked to the stellar differential rotation and is largely driven by shearing at the tachocline—a narrow boundary layer separating the convective and radiative zones (Charbonneau 2014). Details of this complex hydromagnetic process are far from being settled even for the Sun, and possibility of other dynamo effects operating elsewhere in the solar interior has been discussed (e.g. Brandenburg 2005). The tachocline disappears in mid-M dwarfs, offering a unique chance to explore cool- star dynamo action in a different environment compared to the Sun. In this context, investigation of the surface magnetism of M dwarfs straddling the boundary of fully convective interior is critically important for guiding development of the stellar dynamo theory. Active M dwarfs is the only class of stars for which magnetic field alters global stellar parameters in an observable and systematic way. Interior structure of these stars is expected to be relatively simple, especially beyond the limit of full convection. Despite this, many studies demonstrated that measured radii of M dwarfs tend to be significantly larger than those predicted by the stellar evolution theory (e.g., Ribas 2006; Torres 2013) and that this discrepancy correlates with the magnetic activity indicators, such as the Ca H&K and X-ray emission (Lo´pez- Morales 2007; Feiden and Chaboyer 2012; Stassun et al. 2012). The leading hypothesis explaining the apparent inflation of M-dwarf radii is a modification of the convective energy transport, governing the interior structure of low-mass stars, by strong magnetic fields (Mullan and MacDonald 2001; Chabrier et al. 2007; MacDonald and Mullan 2014; Feiden and Chaboyer 2013, 2014). Empirical determinations of the surface magnetic field strengths of M dwarfs are therefore instrumental for constraining and testing theoretical models of magnetised stellar interiors. M dwarfs have been recently established as favourable targets for searches of small exoplanets and in-depth studies of their atmospheres. Due to their low mass, M dwarfs exhibit a higher amplitude reflex radial velocity variation compared to a sun-like star orbited by the same planet. Moreover, a lower luminosity of M dwarfs means that habitable zones are located much closer to the central star. This translates to shorter orbital periods and higher radial velocity amplitudes for terrestrial planets residing in those zones (Kasting et al. 2014). All these factors facilitate discovery and analysis of rocky planets in M-dwarf exoplanetary systems. Both ground-based radial velocity searches (Bonfils et al. 2013) and surveys of transiting exoplanets from space (Dressing and Charbonneau 2015) confirm existence
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